Although the high capacitance and high power density characteristics of ultracapacitors endorses its feasibility in electric vehicle applications, the energy capacity limitation dictates the need for a much higher energy sustainable source, namely a battery bank. The objective of integrating batteries and ultracapacitors is to create an energy storage system with the high
energy density attributes of a battery and the high power density of an ultracapacitor. In essence, the goal is to exploit the advantages of both the devices through ultracapacitor hybridisation of the two technologies in a vehicular power system architecture.
Ozatay et al. [59] concisely described this hybridisation as emulating a “non-existent super- device” by coordinated power transfer of batteries and ultracapacitors. An illustration of power density versus energy density of existing electrical storage devices is shown in Figure 2.5.
10-1 100 101 102 103
10-2 10-1
100 101 102 103 104 105 106
"Super-Device"
created by controlled power distribution Electrolytic capacitors
Electrochemical capacitors Lead acid batteries
Lithium ion batteries NiMH batteries
Ni Cad.
batteries Approximate limits of m
anufacturin
g technology
Energy density [Wh/kg]
Power density [W/kg]
Figure 2.5 Power Density versus Energy Density of current energy storage technologies (Reproduced from Ozatay et.al [59])
The proposition of combining high power density ultracapacitors with high energy density batteries was first claimed in 1992 by Michio Okamura [60]. However, the development of ultracapacitor systems specifically for vehicular applications only began in recent years. In 1996, Burke [46] produced a report on the prospective usage of ultracapacitors in electric and hybrid electric vehicles. Following this, various authors have examined the hybridisations of batteries and ultracapacitors in vehicle power systems.
The most basic method in combining an ultracapacitor system with a battery system as a power supplement device is to simply connect both systems in parallel. This has to be done with particular consideration to the maximum terminal voltage of the ultracapacitor system.
Spyker and Nelms [61] looked at predicting the run-time of a ultracapacitor in a simplified model consisting of a battery and ultracapacitor connected in such a parallel configuration.
For the arrangement shown in Figure 2.6, the authors concluded that the ultracapacitor is only suitable for low duty cycles, as the battery current will surpass the capacitor current during long pulse durations.
DC/DC Rb
Rc
Ultracapacitor
RLoad Battery
Figure 2.6 Battery and Ultracapacitor supplying a constant power load (Extracted from [61])
In the topology shown in Figure 2.6, the battery potential determines the maximum discharge ability of the ultracapacitor. This direct interfacing of the battery and ultracapacitor is achieved by initially pre-charging the ultracapacitor to a terminal voltage of equal magnitude to the battery open circuit voltage prior to making the parallel connection.
Following this, any current division between the battery and ultracapacitor is determined purely by the two branch resistances. Even though it would appear less efficient in terms of discharge capacity, the study provided a basic idea for other researchers to work on. For the same configuration, Miller [5] provides an analysis for the optimum sizing of the ultracapacitor and battery system for a 1610 kg mid-size passenger vehicle. The direct parallel connection of ultracapacitors and batteries are said to be in a passive configuration since there is no external intervention of power sharing between the devices.
Gao, Dougal and Liu [62, 63] produced comparison data of active and passive power sharing between ultracapacitors and batteries under varying load conditions. Figure 2.7 illustrates the circuit configurations. Using their Virtual Test Bed (VTB), the authors simulated and
experimentally verified an increase of power deliverability with the active configuration. In the passive system, the power sharing capabilities of the devices were dictated by the impedance of the components themselves.
Passive Configuration
DC -DC
Active Configuration
Figure 2.7 Ultracapacitor- Battery systems. Passive and Active Configurations (Reproduced from Gao et al [63])
Patterson [64] classified the active configuration possibilities of batteries and ultracapacitors into two types. The first type has the ultracapacitor connected directly across the DC bus and the battery connected through a bi-directional DC-DC converter. The second configuration has the battery on the main DC bus instead. The two configurations are shown in Figure 2.8.
Inverter Bi-
directional DC/DC
Batt UCap
Inverter Bi-
directional DC/DC
UCap Batt
(a) (b)
Figure 2.8 Connection configurations of ultracapacitors to an EV propulsion system (Reproduced from Yan and Patterson [64])
Mellor, Schofield and Howe [65] also examined both these configurations and stated that having the ultracapacitors connected directly to the inverter as shown in Figure 2.8 (a) is likely to yield high efficiency. Notably, in the configuration of Figure 2.8 (a), the entire battery power has to be transferred through the DC-DC converter and hence lowering the
Trở kháng
need for precise energy management to ensure the effectiveness of system energy content.
As an initial plan, the authors proposed the following as a simple idealised energy management scheme. Here the buffer unit refers to ultracapacitors.
• “The buffer unit normally supplies the peak power”
• “The battery supplies the average power”
• “If the DC link falls below a minimum set level, all further power requirement is drawn directly from the battery”
• “If the buffer unit is fully charged, any regenerative energy is diverted to the battery”
One of the key benefits of integrating ultracapacitors with batteries in an electric vehicle propulsion system is the extra ability to harness regenerative energy. Steiner and Scholten [66] demonstrated the potential of using ultracapacitors as an energy recovery system in larger DC fed applications. By harnessing regenerative energy in a railway vehicle application, the authors expected to increase energy savings by 30%. This gain is possible for vehicles with very large peak to average power ratios and extended regenerative braking events. For road vehicles, the figures are lower and are heavily influenced by vehicle drive cycles and overriding functions such as anti-lock braking, which pre-empts regenerative braking modes [5].
A simulation study of regenerative energy handling by Dixon, Ortuzar, and Wiechmann [67]
was reported with very promising energy recovery results. The authors proposed connecting a series of ultracapacitors through a single Buck-Boost converter, which was then paralleled to a battery pack (see Figure 2.9). As a control scheme, they suggested a primary control loop to establish the ultracapacitor current reference and a secondary control loop to generate the required PWM signals for the Buck-Boost converter. Subsequently, Dixon’s team constructed a prototype vehicle to implement their battery and ultracapacitor system for field-testing [68]. The authors reported an 87% energy efficiency with opportunity for improvement.
Propulsion Load
Buck
Boost
Supercapacitors Batteries
Figure 2.9 Ultracapacitor- Battery system with a Buck-Boost converter (Extracted from Dixon et al. [68])
Also using cascaded proportional loops, Ozatay et at. [59] experimented on a frequency – based separation of battery and ultracapacitor currents in their test vehicle. In essence, the authors used a variable bandwidth low-pass filter for the battery current and a band-pass filter for the ultracapacitor current. Results of their drive cycle simulations showed a reduction in battery current stresses but not all peak currents were suppressed. The authors concluded that additional focus on energy management is required to achieve higher ultracapacitor efficiencies and battery life.
Baisden and Emadi [51] demonstrated a control strategy based on selecting three operating modes of a DC-DC converter to determine the power split between an ultracapacitor bank and battery pack. Referencing a dynamic variable to a look-up table, the approach showed that high current stresses experienced by the battery pack could be reduced by blending the power contribution with an ultracapacitor bank. During regenerative power cycles, the operation of the converter chargers the ultracapacitor bank to maximum state of charge and then diverts the access power to the battery pack. Using the Advanced Vehicle Simulator (ADVISOR)[69] package, the authors showed that the hybridisation of battery and ultracapacitors allowed the battery pack to be downsized to 70%. Although ultracapacitors were added to the energy storage system, the significant reduction in battery mass plus the increase in battery life justifies the addition of 35 ultracapacitor cells.
a band-pass
Arnet and Haines [70] designed a high power ultracapacitor interface-converter based on a high level view of energy flow. The authors developed a two-quadrant buck boost converter following an “intelligent” description of how the system should work. Flow of energy was represented by the following statements:
• “The primary source (battery or fuel cell) covers the average power consumption”
• “If more power is needed, energy is drawn from the ultracapacitors”
• “If less power is required, energy can be stored back into the ultracapacitors”
Along with the need to perform voltage and current regulation, the approach used by the authors provided a first estimation in describing the required tasks of the converter through high-level system definitions. A similar top- down approach has been adopted in the scope of this project to generate a strategic power and energy management system.
In a study supported by the U.S Department of Environment, Pay and Baghzouz [71]
described the criteria for the coupling of batteries and ultracapacitors concisely. They stated that the battery current has to be maintained as constant as possible with slow transitions from current levels during load transients. In unison, the ultracapacitors have to charge as fast as possible during regenerative cycles and “discharge most of its stored energy during acceleration”. In order to control the power throughput of the ultracapacitor bank, some form of regulation is necessary. This implies that any form of coupling that restricts the power transfer from the ultracapacitor system is not acceptable. This relates to the passive configuration described by Gao et al. [63] (Figure 2.7) since in the passive configuration, power flow in and out of the ultracapacitor is bounded by the battery terminal voltage. In their concluding remarks, the authors of [71] commented that, “ The best control strategy is not fully developed due to challenging control issues ”. The specifics of these issues and the techniques to integrate and manage the power flow and energy content are still a challenging research question.
In general, literature shows the need for an intermediate power electronics converter to regulate the power flow between the load, battery and ultracapacitor. Schupbach and Balda [72] produced a comparison study of three DC-DC converters for a hybrid electric vehicle
Text
system. They compared the component design ratings of a Half bridge, Cuk and the SEPIC (single-ended primary inductance converter)/ Luo combined converter. Each topology had its advantages as well as disadvantages in terms of size, thermal constraints, conduction losses and isolation. Recognisably, the design of a converter to handle the wide operating voltage of an ultracapacitor bank will not be straightforward. In order to extract as much energy out of a series of ultracapacitors, the current handling capability of the converter will need to be very high.
Instead of a battery pack as a primary energy source, Drolia, Jose and Mohan [73] examined connecting ultracapacitors to fuel cells via a switched mode converter. Compared to other cited literature, the authors aimed at deep discharging the ultracapacitor bank. Their work can be extended to a battery-ultracapacitor configuration since both fuel cells and batteries have slow dynamic responses. In a fuel cell system, an inrush current can cause a total system shutdown whereas with batteries, a thermal rise and lifespan reduction can be expected.
Ohkawa’s proposal [74] to augment its fuel cell vehicle (Honda- FCV) with ultracapacitors employs a DC-DC converter on the fuel cell inputs. Called the VCU (Voltage and current Control Unit), the interface controls power flow from the fuel cell to the propulsion system DC bus. The ultracapacitors were however connected directly to the bus via double-pole relays. Power flow control is established using two low pass filters that are used to regulate a single power electronic stage on the fuel cell side. Although this interface topology and control technique increases the system’s receptivity to regenerative power, the useable energy of the capacitor is still restricted and bounded by the direct DC bus connection.
For a topology that provides galvanic isolation, Chiu and Lin [75] described an arrangement to interface a low voltage battery pack to a high voltage fuel cell and DC bus. Their topology offers the benefit of smaller power inductor values compared to conventional single stage designs, however two primary side inductors and an isolation transformer is required. The arrangement is directly applicable to ultracapacitor-battery hybrids with ultracapacitors operating on the low voltage side but requires the battery to be continuously
converter
converter proposed by Todorovic et.al [76] uses cascaded boost converters to attain a possible 2:1 voltage variation on the primary energy source. Their design to interface a fuel cell stack with ultracapacitors provides a stiff and isolated output voltage while allowing a wide input voltage swing on fuel cell voltage. The ultracapacitor in this topology services the positive peak power demands. However, regenerative power handling capability is not inherent. The concepts of both these topologies are in fact derivations of the well established Weinberg converter [77], which dates back to 1974.
For a HEV application, Cegnar, Hess and Johnson [78] designed a mild hybrid system using only ultracapacitors as the energy storage system. The design relied on regenerative braking as the sole source for cyclically charging the ultracapacitor bank. No batteries were used in their design. To achieve voltage stiffness, they used a high and low voltage ultracapacitor bank coupled with a boost converter. Though not explicitly stated in their report, it is likely that such a design will require a very large inductor. This large inductor would be necessary to transfer power from the ultracapacitors to the load while maintaining the output voltage specification. However, their concept and simulation results interestingly show the capability of the ultracapacitors in recapturing large regenerative currents of magnitudes exceeding 200A. Early studies of using ultracapacitors as the sole energy storage device in HEVs in fact began during the early development of ultracapacitor technology itself. In 1994, Farkas and Bonert [79] examined the possibilities of replacing batteries with ultracapacitors if battery technology does not progress in terms of power deliverability.
Reports have shown convincing facts that the high power stress of a battery pack can be mitigated to a bank of ultracapacitors. Hence a method to determine the number of ultracapacitor cells, the capacity of each cell and the physical connections is needed.
According to Schupbach et al [80], the sizing of a ultracapacitor bank was conventionally estimated by dividing the vehicle load energy and load power requirement by the power and energy coefficients of the ultracapacitor. Since these coefficients vary with respect to the load power levels, a more accurate sizing method was proposed. To capture this non- linearity and with the optimisation goal to minimize weight and volume, the authors implemented an iterative sizing procedure using the gclsolve optimisation routine developed by TOMLAB [81].
2.6 Ultracapacitor augmentation issues In investigating the details of ultracapacitor based energy storage systems, several circuit implementation problems were found. The predominant limitation of ultracapacitor technology is the single cell voltage limitation, which is currently 2.5 Volts. Because of this limitation, multiple cells are connected in series to achieve a higher terminal voltage. Doing so introduces a cell equalisation or balancing issue that is critical for both component failure prevention and energy storage utilisation. On this subject, researchers investigated several cell-balancing techniques. Linzen et al. [82] investigated four different cell-balancing topologies. In essence, the authors of looked at both passive and active cell balancing techniques and reported that a DC-DC converter type topology would be a practically unattractive solution.
Barrade’s et al. [83] voltage sharing device used an inductor, which was switched between adjacent cells via a pair of transistors. The configuration, based on buck-boost converter topology, demonstrated the achievability of voltage balancing through some form of active switching methods. Contrary to the conclusions of Linzen’s et al. [82], Barrade’s [83] DC- DC based cell equaliser showed a practical feasibility. Along the same subject, Barrade in [84] investigated the reduced energy storage capability of series connected ultracapacitor cells if the voltage levels are not shared equally. In a recent (patent pending) design [58], Miller and Everett [85], developed a non-dissipate charge equalisation circuit to address this voltage sharing problem. Also based on active switching techniques, Miller’s circuit was designed for a 15V bank of ultracapacitors specifically for the automotive industry
2.7 Alternative ultracapacitor system configurations
Typically, ultracapacitor banks have been designed as a fixed series of cells to satisfy the terminal voltage demand. Multiple series strings can then be connected in parallel to increase the energy storage capacity of the ultracapacitor bank. The fixed bank of ultracapacitors is then coupled to a DC/DC converter that facilitates control of power flow. Moving away from the fixed configuration topologies, Okamura [60] stipulated that a bank switching topology is capable of achieving a 40% increase in usable energy of a ultracapacitor storage
system. Theoretically, the switching of the ultracapacitors to closely match the terminal voltage is similar to synthesising a current pump by controlling a voltage source. Based on this concept, Takara et al [86, 87] simulated series/parallel bank switching on the premise that the usable energy that the ultracapacitors are able to provide is increased. This was done without the use of a designated DC-DC converter. Following this, Rosario, Economou and Luk [88] reported that since the peak power demands of propulsion load in an electric vehicle are relatively of short intervals, sequential switching of ultracapacitors could be exploited. By coordinating the switching topology, the effective energy that can be extracted out of the ultracapacitor network showed an increase whilst the terminal voltage constraints were satisfied by sequentially changing the connections within the ultracapacitor network.
Miller and Everett [85] studied the effects of ultracapacitor time constant in relation to the specific demands of non-propulsion loads. They introduced the concept of distributing banks of ultracapacitors throughout the vehicle power network. By matching the capacitance to the load power and demand frequency, they demonstrated an increased utilisation of the energy content in smaller, matched capacity ultracapacitor banks. Further to this, distributing the ultracapacitors also eliminates the single point of failure that can occur in a single ultracapacitor bank configuration. The results of their report are of particular interest, since it supports the concept of adapting the capacitance of an ultracapacitor network to the load profile, this concept that was also investigated by Rosario, Economou, Luk and El-Hasan [89] in a publication on pulse power management.
2.8 Observations and Hypothesis
Observations
The fundamental task of a strategic power and energy management system in the context of electric vehicles is to control and coordinate the power generation, energy storage and subsystems power flow for maximum overall system efficiency. Although the generation and storage systems have their specific optimal operating ranges in terms of power and energy output efficiency, an overall systems approach to the coordinated operation is called for. The optimisation problem of power and energy management is in fact a global one [90].
Considering the problem on the basis that global optimality results from individual subsystem optimality may not be accurate.
A unified power and energy management system has the challenging mission of handling several tasks, which may be subdivide as follows: -
• Energy resource and power flow coordination
• Power generation and peak power handling
• Power availability for propulsion and safety critical loads
• Power quality and stability
• Regenerative power handling
Essentially, electric load management in a vehicle can be divided as shown in Figure 2.10
Vehicle Electrical Load
Propulsion Load
Non Propulsion Load
Figure 2.10 Vehicle electric load classification